Medical imaging modalities, such as MRI, X-ray, gamma scintigraphy, and CT scanning, have become extremely important tools in the diagnosis and treatment of various diseases and illness. Imaging of internal body parts relies on the contrast between the targeted organ and the surrounding tissues. The targeted organs or tissues are visible by the use of a particular metallopharmaceutical contast agent. In X-ray diagnostics, increased contrast of internal organs, such as kidney, the urinary tract, the digestive tract, the vascular system of the heart, tumor, and so forth is obtained by administering a contrast agent which is substantially radioopaque. In conventional proton MRI diagnostics, increased contrast of internal organs and tissues may be obtained by administrating compositions containing paramagnetic metal species, which increase the relaxivity of surrounding protons. In ultrasound diagnostics, improved contrast is obtained by administering compositions having acoustic impedances different than that of blood and other tissues. In gamma scintigraphy, improved contrast of internal organ is obtained by the specific localization of a radiopharmaceutical.
Attachment of metal ions to biomolecules such as antibodies, antibody fragments, peptides, peptidomimetics, and non-peptide receptor ligands leads to useful target-specific diagnostic and therapeutic metallo-pharmaceuticals. These include fluorescent, radioactive and paramagnetic metal ions attached to proteins that can be used as probes in vivo in biological systems and in vitro in analytical systems as radioimmunoassays. For example, attachment of radionuclides to monoclonal antibodies that recognize tumor associated antigens provides radioimmunoconjugates useful for cancer diagnosis and therapy. The monoclonal antibodies are used as carriers of desired radioisotope to the tumor in vivo.
Radiopharmaceuticals can be classified into two primary classes: those whose biodistribution is determined exclusively by their chemical and physical properties; and those whose ultimate distribution is determined by receptor binding or other biological interactions. The latter class is often called target-specific radiopharmaceuticals. In general, a target specific radiopharmaceutical can be divided into four parts: a targeting molecule, a linker, a bifunctional chelator (BFC), and a radionuclide. The targeting molecule serves as a vehicle which carries the radionuclide to the receptor site at the diseased tissue. The targeting molecules can be macromolecules such as antibodies; they can also be small biomolecules (BM) such as peptides, peptidomimetics, and non-peptide receptor ligands. The choice of biomolecule depends upon the targeted disease or disease state. The radionuclide is the radiation source. The selection of radionuclide depends on the intended medical use (diagnostic or therapeutic) of the radiopharmaceutical. Between the targeting molecule and the radionuclide is the BFC, which binds strongly to the metal ion and is covalently attached to the targeting molecule either directly or through a linker. Selection of a BFC is largely determined by the nature and oxidation state of the metallic radionuclide. The linker can be a simple hydrocarbon chain or a long polyethylene glycol (PEG), which is often used for modification of pharmacokinetics. Sometimes, a metabolizeable linker is used to increase the blood clearance and to reduce the background activity, thereby improving the target-to-background ratio.
The use of metallic radionuclides offers many opportunities for designing new radiopharmaceuticals by modifying the coordination environment around the metal with a variety of chelators. The coordination chemistry of the metallic radionuclide will determine the geometry of the metal chelate and the solution stability of the radiopharmaceutical. Different metallic radionuclides have different coodination chemistries, and require BFC's with different donor atoms and ligand frameworks. For “metal essential” radiopharmaceuticals, the biodistribution is exclusively determined by the physical properties of the metal chelate. For target-specific radiopharmaceuticals, the “metal tag” is not totally innocent because the target uptake and biodistribution will be affected by the metal chelate, the linker, and the targeting biomolecule. This is especially true for radiopharmaceuticals based on small molecules such as peptides, due to the fact that in many cases the metal chelate contributes greatly to the overall size and molecular weight. Therefore, the design and selection of the BFC is very important for the development of a new radiopharmaceutical.
The same principle used for target-specific metalloradiopharmaceuticals also applies to target-specific MRI contrast agents and ultrasound agents. Unlike the target-specific metalloradiopharmaceutical, where excess unlabeled biomolecule can compete with the radiolabeled-BFC-biomolecule conjugate and block the docking of the radiolabeled receptor ligand, MRI and ultrasound contrast agents contain no excess BFC-biomolecule conjugate. Saturation of the receptor sites will maximize the contrast between the diseased tissues and normal tissue provided that the use of a relatively large amount of metal-BFC-biomolecule complex does not cause unwanted side effects.
Several BFC systems such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepetaacetic acid (DTPA), as well as their derivatives, have been reported to form thermodynamically stable metal chelates when attached to proteins. However, in vivo instability of the radioimmunoconjugate or the chelate under physiological conditions results in the breakdown of these complexes. Hence, there is a continuing need for new BFC's with a macrocyclic ligand framework for the radiolabeling of biomolecules such as antibodies, antibody fragments, peptides, peptidomimetics, and non-peptide receptor ligands.
For a therapeutic radiopharmaceutical or an MRI contrast agent, it is especially important to keep the metal chelate intact under the physiological conditions, particularly in the presence of native chelators, such as transferrin, which have very high affinity for trivalent lanthamide metal ions. This requires the chelant to form a metal chelate with thermodynamic stability and kinetic inertness. Macrocyclic chelants with three-dimensional cavities are of particular interest because they form metal complexes with high stability. They often exhibit selectivity for certain metal ions based on metal size and coordination chemistry, and capability to adopt an preorganized conformation in the uncomplexed form, which facilitates metal complexation.
Polyaza macrocycles have been widely used as chelants for a variety of transition metals. The macrocyclic polyaminocarboxylates such as 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetracetic acid (DOTA) and 1,4,8,11-tetraazacyclo-tetradecane-1,4,8,11-tetracetic acid (TETA) are known to form highly stable metal complexes due to their highly preorganized macrocyclic ligand framework. Their Gd complexes have been widely used as MRI contrast agents. Examples include gadolinium complexes Gd-DOTA (Dotarem™, Guerbet/France), Gd-HP-DO3A (ProHance™, Bracco/Italy), and Gd-DO3A-butrol (Gadovist™, Schering/Germany). These macrocyclic chelants have also been used as BFC's for the radiolabeling of proteins and peptides with various diagnostic and therapeutic radionuclides (such as 111In and 90Y). In all those cases, the linkages between N-donors of the macrocycle are either ethylene- or propylene-bridges.